This invention relates to a means for providing gravity stabilization of an object such as an underwater pipeline, and to a method of installing such means.
It is usually necessary to stabilize an underwater pipeline both during installation of the pipeline and once the pipeline is in position on the floor of the body of water. During installation of the pipeline, the gravity stabilization is required to overcome any buoyancy effects of the pipeline. Once the pipeline is in position, gravity stabilization is required to prevent the pipeline from shifting under the effects of wave motion and water currents.
In some situations gravity stabilization is achieved by providing a coating of concrete on the pipeline so as to increase its weight; this procedure is known as weight-coating. It is not altogether satisfactory as some of the increased weight of the pipeline is utilized in offsetting the buoyancy effect of the increase in diameter of the pipeline.
In other situations, gravity stabilization is achieved by use of a massive mat structure comprising concrete elements flexibly linked together to allow the mat structure to be draped over the pipeline with the outer edges of the mat structure resting on the floor of the body of water. While such mat structures perform satisfactorily once in position on a pipeline, they are designed to be installed on a section of the pipeline only after the pipeline section has been laid and so they do not satisfy the need for stabilization of the pipeline section during laying. Furthermore the pipeline remains in a vulnerable state on the floor of the body of water until the mat structures have been laid. This can be a problem particularly if the site has to be evacuated owing to adverse weather conditions before all of the mat structures have been installed. Additionally, installation of the mat structures presents some difficulty as they have to be supported in a deployment frame while they are lowered into position above the appropriate section of the pipeline and then carefully located in position.
It is also known to provide a ballast weight assembly for a pipeline laid on marshy ground or underwater. The ballast weight assembly is adapted to clampingly engage the pipeline and comprises a pair of clamps pivotally connected together at their upper ends by a massive saddle comprising two sections which are pivotally interconnected. The lower or free ends of the clamps are interconnected by a flexible band. The ballast weight assembly is installed onto a pipeline using a detachable support beam to lower it onto the pipeline with the clamps located one on each side of the pipeline. As the ballast weight assembly is lowered onto the pipeline, the flexible band deflects as it comes into contact with the upper section of the pipeline and so causes the clamps to pivot inwardly and clampingly engage the pipeline to secure the assembly in position. Because of the massive nature of the saddle, the ballast weight assembly would appear to provide the desired anchoring function.
There are, however, several deficiencies which appear to exist with the ballast weight assembly. One such deficiency is that the clamping action on the pipeline is very much reliant on vertical loadings applied to the clamps by the massive saddle. It therefore follows that the saddle should not rest on the pipeline for otherwise the vertical loadings on the clamps would be reduced with the result that the clamping action on the pipeline would diminish. A further deficiency is that massive saddle produces a high center of gravity in the ballast weight assembly with the result that the center of gravity is above the center of the pipeline when the assembly is in position. This is detrimental to the rotational stability of the ballast weight assembly when on the pipeline and means that it is most unlikely that the ballast weight assembly could be installed on a pipeline being lowered onto the seabed under normal operational conditions at sea. The rotational instability created by the ballast weight assembly may also crease problems for a pipeline installed underwater owing to wave and current action. A still further deficiency arising from the massive saddle is the large profile that the ballast weight assembly presents when in position on a pipeline. As a result of the large profile, there is a substantial surface area of the ballast weight assembly exposed to the influences of underwater currents and other water motion. The forces which are imparted to the ballast weight assembly as a result of the influences of such water motion can cause the assembly to dislodge from the pipeline or the pipeline to shift laterally underwater.
From the foregoing discussion it is evident that the means for gravity stabilization which have been described have not performed altogether satisfactorily in the situations outlined.
The present invention seeks to provide a novel and useful means for gravity stabilization which when applied to stabilization of underwater pipelines can be utilized to provide gravity stabilization both during laying of the pipeline and once the pipeline is in position on the floor of the body of water.